The incandescent lamp is the primary luminary for household and residential lighting. This lamp generally includes an incandescent filament within a predetermined non-oxidizing atmosphere which is contained within a tear-drop shaped envelope and mounted, for example, within an Edison-type base which is screwed into a permanent fixture or into a movable socket.
Despite their widespread use, incandescent lamps are relatively inefficient, producing only 15-17 lumens per watt of input power and have relatively short, unpredictable service lives. Fluorescent lamps, which have efficiencies as high as 80 lumens per watt, provide an attractive alternative to incandescent lighting. Conventional fluorescent lamps, however, require a long tubular envelope which, together with the need for auxiliary ballasting equipment, has somewhat limited their acceptance in the home lighting market. Increased residential use of fluorescent illumination, with attendant savings of energy, can be achieved from the development of fluorescent lamps which are directly compatible with existing sockets and incandescent lamp fixtures.
U.S. patent application Ser. No. 642,142, now issued U.S. Pat. No. 4,017,764, by John M. Anderson, filed Dec. 18, 1975 and assigned to assignee of this invention, describes an electrodeless fluorescent lamp adapted for economical substitution in place of existing incandescent lamps which comprises an ionizable medium contained within a phosphor coated globular envelope. A closed loop magnetic core, which may be ferrite, is contained within the ionizable medium to induce an electric field, ionize the medium, and stimulate visible light output. The core is energized by a radio frequency power source in the lamp base structure. Current flows from the power supply through a multi-turn winding on the core and induces a single turn current flow in a diffuse plasma linking the core.
U.S. patent application Ser. No. 642,056, allowed by the Patent and Trademark Office on Aug. 4, 1976, now issued U.S. Pat. No. 4,005,330, which is assigned to the assignee of this invention, describes a similar electrodeless fluorescent lamp structure wherein a closed loop magnetic core is disposed through a central tunnel in a substantially globular fluorescent lamp. Current flow in the lamp is induced by a radio frequency power source in the manner described above.
The above-described electrodeless fluorescent lamps are highly efficient and substantialy free from electromagnetic radiation problems which characterized prior art, open-core induction ionized fluorescent lamp designs. In lighting installations which comprise large numbers of such lamps, however, the sum of the electromagnetic radiation produced may, in some cases, produce a source of electromagnetic interference (EMI) or exceed applicable government or industry standards. It is, therefore, desirable to reduce the radiation of electromagnetic energy from induction ionized lamp structures.